With the recent advancement of informatization, there has been a growing demand for data communication traffic for data communication. Thus, an increase in the communication bands of networks and a reduction in operation costs have been demanded.
Mobile phone networks and the like utilize a network formed of radio links, such as an FWA (Fixed broadband Wireless Access) network that uses a radio scheme utilizing frequencies such as in a millimeter waveband at which wide-band transmission can be carried out.
FIG. 1 shows a graph of the correlation between the SNRs (Signal to Noise Ratios) and transmission rates of modulation modes. In the graph in FIG. 1, the axis of abscissas indicates the SNR, and the axis of ordinate indicates a bandwidth. The communication quality of the radio link varies depending on the SNR of the reception signal. Thus, much attention has been paid to an adaptive modulation technique in order to further increase the bandwidth of radio links.
The adaptive modulation technique adaptively finds and uses a modulation scheme with the highest transmission efficiency based on the status of radio communication via radio links. The adaptive modulation technique allows the optimum radio communication to be carried out in accordance with the radio environment. This enables frequency efficiency to be improved.
Provided that a modulation mode to be used by each radio link in the future can be predicted, the communication quality of traffic flowing through the network can be guaranteed. Examples of a method for predicting a modulation mode are disclosed in JP2006-505221A (hereinafter referred to as Document 1) and JP2004-363679A (hereinafter referred to as Document 2).
As disclosed in Document 1, a radio link using the adaptive modulation technique may have a certain level of correlation with the past history. Thus, the modulation mode to be used by the radio link can be predicted (estimated). Moreover, as shown in Document 2, the modulation mode can be predicted (estimated) by utilizing weather information.
An example of path control performed in a radio network is disclosed in M. S. Kodialam et al., “Minimum interference routing with applications to mpls traffic engineering”, INFOCOM, 2000 (hereinafter referred to as Document 3).
If the path control disclosed in Document 3 is applied to an FWA network that uses the adaptive modulation technique, then unlike in the case of wired communication, the communication rate of the radio link will be varied by the adaptive modulation in connection with the path control. Thus, congestion or the like may occur to degrade the communication quality of traffic.
On the other hand, JP2006-313992A (hereinafter referred to as Document 4) proposes a load distribution technique for an FWA network that utilizes the adaptive modulation. A method disclosed in Document 4 is a scheme in which traffic is distributed among radio links in a plurality of routing paths for relay transmission. The use of this scheme allows the traffic to be distributed, enabling the utilization rate of the whole network to be improved.
However, in this scheme, packets may pass through different paths, resulting in possible jitter due to delay or the like. In this case, the scheme is not suitable for traffic such as VoIP (Voice over IP) for which communication quality needs to be guaranteed. Such traffic requires path control that allows the traffic to pass through the same path. Furthermore, if the scheme is utilized without any modification, since the path cannot be specified, it is difficult to estimate what degrees of priority will be required to maintain the quality of the traffic. As a result, maintaining and managing the communication quality of priority traffic is difficult.
An example of an existing technique with the path control taken into account is disclosed in J. Sangiamwong et al., “QoS-based adaptive modulation under rainfall environment in gigabit millimeter-wave broadband wireless access system,” IEEE RAWCON, 2003 (hereinafter referred to as Document 5).
Document 5 proposes a path control scheme for an FWA network with an adaptive modulation function. The scheme proposed in Document 5 searches for a modulation mode and a path which are optimum for each flow. However, this scheme is path control based on the current radio link status. Thus, the modulation mode cannot be ensured to be maintained in the future. Furthermore, in each base station, the modulation mode is selected for each flow, disadvantageously resulting in a significant overhead.
As described above, a technique is required in which the modulation mode to be used by each radio link that is varied by the adaptive modulation is predicted or estimated to allow path control to be performed.